1. Field of the Invention
The present invention relates to an optical signal processor for performing an arbitrary advanced optical filtering treatment in the fields of optical communication, optical exchange and optical computing, as well as a method of controlling the processor, a method of its designing, and a method of its production.
2. Description of the Prior Art
In recent years, wide attention has been drawn to optical signal processors capable of filtering optical signals, as such, in broad bands at high speeds, without converting them into electric signals, in the fields of optical communication, optical exchange and optical computing. In optical frequency multiplex communication where optical signals are multiplexed and transmitted, in particular, optical frequency filters which filter incoming frequency-multiplexed optical signal according to different frequencies are parts of importance.
Optical signal processors that have been reported to attain the above purpose include one of a transversal type as shown in FIG. 1 (see Japanese Patent Application Laying-Open No. 212822/1990). This type of device is constructed such that variable directional couplers 3-1 through 3-N are disposed on the input ports 5, 6 side, optical signals are distributed to branching waveguides 10-1 through 10-N at arbitrary branching ratios, and the branched optical signals are combined by a coupling part composed of N-1 3 dB directional couplers 9-1 to 9-(N-1). On the branching waveguides 10-1 through 10-N are arranged phase controllers 4-1 through 4-N for controlling the phases of the branched optical signals individually. This type of device can achieve the desired transmission characteristics by varying the coupling rates of the variable directional couplers 3-1 through 3-N, while varying the amounts of phase shift of the phase controllers 4-1 to 4-N.
According to the above-described circuit, the functional accuracy of the filter characteristics can be improved by increasing the number of the branching waveguides. Increasing the number N of the branching waveguides for the purpose of increasing the filter accuracy, however, poses the problem of the maximum transmittance of the transmission characteristics becoming small. This is because the directional coupler is basically a two-input two-output element, so that the 3 dB directional couplers 9-1 through 9-(N-1) of the coupling part have corresponding dummy ports 11-1 through 11-(N-1), from which the optical signals are partly released.
An optical signal processor of an improved transversal type as shown in FIG. 2 (see Japanese Patent Application Laying-Open No. 11226/1993) has been reported as a device for overcoming the above-mentioned drawback. In the aforementioned first transversal type device, the variable directional couplers 3-1, 3-3, 3-5 to 3-(2N-1) were used only on the input port side to optimize the coupling rate, while the 3 dB directional couplers 9-1 through 9-(N-1) were used at the coupling part on the output port side. In this second device, by contrast, variable directional couplers 3-2, 3-4, 3-6 to 3-2N are used not only on the input port side, but also on the output port side, to optimize the coupling rate. Because of this construction, the second conventional device has succeeded in curtailing the lowering of the optical intensity after filtering that is noted when the number N of the branching waveguides is increased.
In the second conventional example, however, the transversal configuration made the number of the dummy output ports practically equal to that in the first conventional example. No matter how optimal the configuration adopted was, release of optical signals from the dummy output ports was not avoidable, and the achievement of a filtering device with a maximum transmittance of 100% was impossible.